From the beginning, Apple said that the iPhone would ship in June of this year and would be a Cingular Wireless (AT&T Wireless) exclusive phone. Late last month, an unofficial launch date of June 11 was reported by CNET. The launch would coincide with Apple's Worldwide Developers Conference in San Francisco which is slated to run from June 11 through June 15.
A timely release is important for them, especially considering they are planning to have sold at least 10 million units by the end of next year. At $499-599 a pop, that might be difficult.